The curved lines in this picture are trails of bubbles formed as charged particles force their way through an unstable mixture of neon and hydrogen. They curve in an applied magnetic field, and spiral because they lose energy quickly.
The highlighted track that curves to the right (red) was produced by an electron while its partner, which turns to the left, was produced by a positron . Together they are referred to as an electron-positron () pair that has 'materialized’ from a high energy photon in the electric field of a nucleus. This is often referred to as 'pair-production’.
Notice that the photon that produced this pair comes from the end of a positive track that stopped abruptly; this is an unusual example of the annihilation of a positron in flight, producing two photons. One of these produced our pair.